There are signs the rookie who replaced him at right tackle, Tyron Smith, has a similar makeup.

“I work beside (Smith),” Pro Bowl tight end Jason Witten said, “and I've been beside some tough guys — look at the guy whose spot he's taking: (Colombo) was one of the toughest. I think Tyron has that mentality, like most good linemen do.”

One day after he hyperextended his right knee, Smith didn't participate in practice. But the Cowboys are hopeful their first-round pick can tough it out and start in the opener at the New York Jets on Sunday.

“One of the things we liked about him coming out is he's a tough guy,” Garrett said. “He works hard. He's physical. He has mental and physical toughness. In the past, he's handled situations like this. We expect him to continue with that approach.”

Witten liked the blue-collar work ethic Smith exhibited throughout camp and the preseason.

“He doesn't complain,” Witten said. “It's a grind in camp for all of us, especially for a rookie. But after practice, he's working with DeMarcus (Ware), fighting through things.”

While Smith took mental reps Thursday, untested second-year player Jermey Parnell worked with the first team.

Parnell declined to talk to the media, but Garrett praised his work.

“He did a nice job,” Garrett said. “He got more and more comfortable as he's gotten more reps. He's a very talented guy. He's very quick and athletic.”

Parnell, who the Cowboys signed off the New Orleans Saints' practice squad last year, is a project after playing just one season at Mississippi (as a defensive end) and not at all in high school. He lettered four seasons as a forward for the Rebels basketball team.

Despite the inexperience, Dallas kept Parnell over 2010 sixth-round pick Sam Young because the club likes his size (6-foot-6, 306 pounds), arm length and foot quickness.

“His physical traits are obvious,” Garrett said. “But one of the things you need to see is some football instinct, some football savvy, some understanding of offensive and defensive schemes. And you need to see some compete in them.

“He's gotten better in all of these areas — understanding how to use his physical ability, his length and his quickness and then understanding what we're trying to get done and having a little instinct and competitiveness to him. It's a process for him.”

Translation: He's not ready to face the Jets' fierce defense. Still, Dallas might not have a choice.

“I'm hopeful (Smith can play), but we can't have a pity party,” Witten said. “We've got to have someone step up. That's what good teams do. But he's shown all the signs he's a tough guy. That doesn't get questioned whether he plays or not.